Wire varnishes, or the so-called wire enamels, are an important industrial commodity. One of their major applications is to make enameled wires for use in making coil windings for electrical motors, which constitute an important class of consumer goods affecting our everyday life. A wide variety of polymers, such as polyimide, polyamide, polyamideimide, polyester, polyurethane, etc., have been used in making wire varnishes. In many instances the selection of the polymeric varnish material is dictated by the temperature at which the enameled wires are expected to be used.
Polyurethanes are well-known to provide excellent electrical and mechanical properties, and are relatively inexpensive. Polyurethane varnishes have been widely used in making enameled wires; however, they exhibit relatively inferior heat resistance. Polyurethane coated wires (or varnished or enameled wires) are typically rated below 135.degree. C.; such a low temperature rating makes polyurethane coated wires unsuitable in many important applications. Enameled wires made from modified or unmodified polyurethane also often do not exhibit satisfactory solderability. In many electrical applications, it is highly desirable to have wires which can be soldered without stripping the coating. Solderability can be expressed in several different ways, for example, it can be expressed as the time required to solder a varnished wire; the lower the time required, the better the solderability.
U.S. Pat. No. 3,869,428 discloses an enamel composition containing imide and urethane groups which are the polyurethane reaction product of a hydroxyl compound and an isocyanate compound selected in respect to each other so that at least one thereof contains imide groups and reacted with each other in a stoichiometric ratio of the hydroxyl groups and isocyanate groups. In the '428 patent, excess trimellitic anhydrides are reacted with diamine or amino acid to form imide diacid, to which ethylene glycol, trimethylene glycol or trimethylol propane is then added to cause an esterification between the hydroxyl group and the acidic group in the diacid. The reaction is controlled such that the hydroxyl groups are in excess which would react with diisocyanates. After the reaction, end NCO groups are protected, or blocked, with phenol. In a separate step, trimethylol propane reacts with diisocyanate, and the free NCO groups after the reaction are also blocked with phenol. The two reaction products are then mixed to form the polyesterimide-modified polyurethane varnish disclosed in the '428 patent. Several disadvantages have been observed from the '428 patent. First, because of the requirement of the esterification reaction as an intermediate step, the process disclosed in the '428 patent operates at undesirably high temperatures, in excess of 200.degree. C. Also, the polyesterimide-modified polyurethane does not significantly improve the heat resistance of the final product.
Japanese Pat. App. Laid Open JP-60-3114 discloses a polymeric elastomer prepared by first reacting trimellitic anhydride with excess diisocyanate. Using .alpha.-pyrrolidone to block the unreacted NCO groups, the intermediate product was mixed with polyester polyol or polyether polyol having two hydroxy groups and a molecular weight greater than 10.sup.4. The process disclosed in JP-60-3114 involves a high temperature baking process which often results in nonuniform thickness of the polyurethane film and dimensional instability. Furthermore, the use of high molecular weight polyols (greater than 10.sup.4) substantially lowers the heat resistance of the polyurethane products; it also causes phase separation thus adversely affecting the physical properties of the final products.
U.S. Pat. No. 4,816,544 discloses a polyamideimide elastomer obtained by reaction of carboxyl-telechelic polymer having a molecular weight of 500 to 10,000, aromatic diisocyanate, trimellitic acid anhydride and a dicarboxylic acid having 4 to 18 carbon atoms in a pre-selected proportion. The composition disclosed in the '544 is only useful as an elastomer. Because it exhibits low glass transition temperature, Tg, and low heat resistance, it does not posses the required physical characteristics for use in wire coating and insulation.